1. Field of the Invention
The present invention relates to an electric motor for use with an electric vehicle or the like, and in particular, to a bearing structure which is capable of cooling bearings that support a rotor shaft in such an electric motor to a satisfactory extent.
2. Description of the Related Art
A known drive unit for an electric vehicle is provided with a motor, a casing having the motor received therein, an oil circulation system that circulates oil for cooling the motor in the casing, and a cooling system that cools the oil circulating in the casing by heat exchange, wherein the oil is circulated by way of installation locations of individual bearings that support a rotation shaft of a rotor thereby to cool and lubricate the bearings (see, for example, a first patent document: Japanese patent application laid-open No. 2001-251814).
In addition, a known pump of the type integrally formed with an electric motor has a cooling and lubricating structure that is constructed as follows. That is, the pump of the integral electric motor type has an impeller for pressurizing liquid fuel (oil), a rotor with a rotation shaft for driving the impeller to rotate, and bearings for supporting the rotation shaft of the rotor, wherein the rotation shaft (rotor shaft) of the rotor has an oil hollow bore for introduction of the pressurized oil (liquid fuel) of a low temperature formed therein in coaxial relation therewith, and a radial bore formed therethrough so as to introduce an amount of oil necessary and sufficient for lubrication of the bearings from the oil hollow bore. Further, a cooling nozzle is arranged for injecting cooling air to the bearings from a position remote from the bearings, and the cooling and lubrication of the bearings are carried out by spraying to the bearings a stream of oil mist air, which is produced by forming the oil introduced from the radial bore into a mist by means of the cooling air (see, for example, a second patent document: Japanese patent application laid-open No. H11-1 66497).
Here, for example, the temperature of the bearings in the known drive unit for an electric vehicle with the rotor rotating at a high speed of 10,000 rpm or more is remarkably raised by the heat due to bearing friction loss, the heat that is generated by the electromagnetic loss in the rotor and transmitted through thermal conduction to the bearings by way of the rotation shaft of the rotor, and so on.
In the known drive unit for an electric vehicle, there is the following problem. That is, when a large amount of heat generated by the rotor rotating at high speed is transmitted through thermal conduction to the bearings by way of the rotor shaft, satisfactory cooling of the bearings can not be performed, so the temperature of oil in the vicinity of the bearings rises and the viscosity of oil decreases, thus obstructing smooth rotation of the bearings.
Further, there is another problem that when a predetermined temperature decided by the material of the bearings is exceeded, the reduction or degradation in strength of the bearings occurs, so the reliability of the bearings themselves is impaired. For example, in case where SUJ2, a material for bearings in general, is used as the material of the bearings, it is known that the strength of the bearings is reduced when the temperature of the bearings becomes about 120 degrees C.
In addition, in the cooling and lubricating structure of the known pump of the integral electric motor type, the bearings can be cooled to a satisfactory extent by the provision of the cooling nozzle for injecting cooling air, so the reduction in the viscosity of oil can be prevented, but on the other hand, there is a problem that the electric motor is increased in size and complicated in structure, resulting in an increase in cost.